Re: [PATCH v6 04/26] rust: alloc: implement `Allocator` for `Kmalloc`

[Benno Lossin]

On 03.09.24 13:48, Danilo Krummrich wrote:
> On Fri, Aug 30, 2024 at 02:45:35PM +0000, Benno Lossin wrote:
>> On 30.08.24 00:04, Danilo Krummrich wrote:
>>> On Thu, Aug 29, 2024 at 06:32:42PM +0000, Benno Lossin wrote:
>>>> On 16.08.24 02:10, Danilo Krummrich wrote:
>>>>> +///
>>>>> +/// For more details see [self].
>>>>> +pub struct Kmalloc;
>>>>>
>>>>>  /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
>>>>>  fn aligned_size(new_layout: Layout) -> usize {
>>>>> @@ -36,6 +52,60 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
>>>>>      unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags.0) as *mut u8 }
>>>>>  }
>>>>>
>>>>> +/// # Invariants
>>>>> +///
>>>>> +/// One of the following `krealloc`, `vrealloc`, `kvrealloc`.
>>>>> +struct ReallocFunc(
>>>>> +    unsafe extern "C" fn(*const core::ffi::c_void, usize, u32) -> *mut core::ffi::c_void,
>>>>> +);
>>>>> +
>>>>> +impl ReallocFunc {
>>>>> +    // INVARIANT: `krealloc` satisfies the type invariants.
>>>>> +    const KREALLOC: Self = Self(bindings::krealloc);
>>>>> +
>>>>> +    /// # Safety
>>>>> +    ///
>>>>> +    /// This method has the same safety requirements as [`Allocator::realloc`].
>>>>> +    unsafe fn call(
>>>>> +        &self,
>>>>> +        ptr: Option<NonNull<u8>>,
>>>>> +        layout: Layout,
>>>>> +        flags: Flags,
>>>>> +    ) -> Result<NonNull<[u8]>, AllocError> {
>>>>> +        let size = aligned_size(layout);
>>>>> +        let ptr = match ptr {
>>>>> +            Some(ptr) => ptr.as_ptr(),
>>>>> +            None => ptr::null(),
>>>>> +        };
>>>>> +
>>>>> +        // SAFETY: `ptr` is either NULL or valid by the safety requirements of this function.
>>>>
>>>> You need some justification as to why calling the three allowed
>>>> functions here.
>>>
>>> What kind of justification do I need? Can you please share some more details on
>>> what you think is missing here?
>>
>> So, you are calling a function pointer to an `unsafe` function. This
>> means that through some invariant you have to know what the safety
>> requirements are (otherwise how can you guarantee that this is OK?). You
>> have the invariant that the pointer points at one of the three functions
>> mentioned above. What are the safety requirements of those functions? I
>> would assume that the only one is that `ptr` is valid. So you can use:
>>
>>     // SAFETY:
>>     // - `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc` and thus only requires that `ptr` is
>>     //   NULL or valid.
> 
> I'm fine adding it, but I'd like to understand why you think it's required in
> the safety comment here? Isn't this implicit by being the type invariant?

You are calling a function pointer to an `unsafe` function that takes a
raw pointer. Without this comment it is not clear what the function
pointer's safety requirements are for the raw pointer parameter.

>>     // - `ptr` is either NULL or valid by the safety requirements of this function.
> 
> This is the part I already have.

I kept it to ensure that you also keep it.

>>>>> +        let raw_ptr = unsafe {
>>>>> +            // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed.
>>>>> +            self.0(ptr.cast(), size, flags.0).cast()
>>>>> +        };
>>>>> +
>>>>> +        let ptr = if size == 0 {
>>>>> +            NonNull::dangling()
>>>>> +        } else {
>>>>> +            NonNull::new(raw_ptr).ok_or(AllocError)?
>>>>> +        };
>>>>> +
>>>>> +        Ok(NonNull::slice_from_raw_parts(ptr, size))
>>>>> +    }
>>>>> +}
>>>>> +
>>>>> +unsafe impl Allocator for Kmalloc {
>>>>
>>>> Missing SAFETY comment.
>>>
>>> Yeah, I think we came across this in an earlier version of the series. I asked
>>> you about the content and usefulness of a comment here, since I'd just end up
>>> re-iterating what the `Allocator` trait documentation says.
>>>
>>> IIRC, you replied that you want to think of something that'd make sense to add
>>> here.
>>
>> Oh yeah, sorry I forgot about that.
>>
>>> What do you think should be written here?
>>
>> I think the best way to do it, would be to push this question down into
>> `ReallocFunc::call`. So we would put this on the trait:
>>
>>     // SAFETY: `realloc` delegates to `ReallocFunc::call`, which guarantees that
>>     // - memory remains valid until it is explicitly freed,
>>     // - passing a pointer to a vaild memory allocation is OK,
>>     // - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
> 
> So, we'd also need the same for:
>   - `unsafe impl Allocator for Vmalloc`
>   - `unsafe impl Allocator for KVmalloc`

Yes.

>> We then need to put this on `ReallocFunc::call`:
>>
>>     /// # Guarantees
>>     ///
>>     /// This method has the same guarantees as `Allocator::realloc`. Additionally
>>     /// - it accepts any pointer to a valid memory allocation allocated by this function.
> 
> You propose this, since for `Allocator::realloc` memory allocated with
> `Allocator::alloc` would be fine too I guess.
> 
> But if e.g. `Kmalloc` wouldn't use the default `Allocator::alloc`, this would be
> valid too.

So if `Kmalloc` were to implement `alloc` by not calling
`ReallocFun::call`, then we couldn't use this comment. Do you think that
such a change might be required at some point?

> We could instead write something like:
> 
> "it accepts any pointer to a valid memory allocation allocated with the same
> kernel allocator."

It would be better, if we can keep it simpler (ie only `realloc` is
implemented).

>>     /// - memory allocated by this function remains valid until it is passed to this function.
> 
> Same here, `Kmalloc` could implement its own `Allocator::free`.
> 
> Maybe just "...until it is explicitly freed.".

I don't really like that, since by that any other function could be
meant. Do you need to override the `free` function? If not then it would
be better.

> Anyway, I'm fine with both, since non of the kernel allocators uses anything
> else than `ReallocFunc::call` to allocate and free memory.
> 
>>
>> Finally, we need a `GUARANTEE` comment (just above the return [^1]
>> value) that establishes these guarantees:
>>
>>     // GUARANTEE: Since we called `self.0` with `size` above and by the type invariants of `Self`,
>>     // `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc`. Those functions provide the guarantees of
>>     // this function.
>>
>> I am not really happy with the last sentence, but I also don't think
>> that there is value in listing out all the guarantees, only to then say
>> "all of this is guaranteed by us calling one of these three functions.
>>
>>
>> [^1]: I am not sure that there is the right place. If you have any
>>       suggestions, feel free to share them.
> 
> Either way, I'm fine with this proposal.
> 
>>
>>
>>>>> +    #[inline]
>>>>> +    unsafe fn realloc(
>>>>> +        ptr: Option<NonNull<u8>>,
>>>>> +        layout: Layout,
>>>>> +        flags: Flags,
>>>>> +    ) -> Result<NonNull<[u8]>, AllocError> {
>>>>> +        // SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`.
>>>>> +        unsafe { ReallocFunc::KREALLOC.call(ptr, layout, flags) }
>>>>> +    }
>>>>> +}
>>
>> Oh one more thing, I know that you already have a lot of patches in this
>> series, but could you split this one into two? So the first one should
>> introduce `ReallocFunc` and the second one add the impl for `Kmalloc`?
>> I managed to confuse me twice because of that :)
> 
> Generally, I'm fine with that, but I'm not sure if I can avoid an intermediate
> compiler warning about unused code doing that.

You can just use `#[expect(dead_code)]` for that in the intermediate
patches.

---
Cheers,
Benno